OBJECTIVE To determine the prevalence and intensity of infection with schistosomiasis among school children and to evaluate the efficacy of praziquantel in normalizing the levels of some haematological and biochemical blood constituents. METHODS A total of 346 school children, aged 7-13 years, from El-Kriab primary school, near the El-Seleit irrigation scheme east of Khartoum, Sudan, were screened for schistosomiasis. All 136 children infected with Schistosoma haematobium and/or S. mansoni were clinically investigated and treated with a single oral dose of praziquantel (40 mg/kg body weight). RESULTS In 74% out of 97 children infected with S. haematobium the egg count exceeded 500 eggs/10 ml of urine and they had high level of eosinophilia and leukocyte counts, high globulin and calcium levels and low potassium. The serum total proteins, globulins, albumin and urea showed statistically significant differences between control and patients and between patients before and after treatment, although the levels were within the normal reference range. Praziquantel treatment was highly effective as proved by a (58%) cure rate, a (98%) reduction in egg count and normalization of the examined blood parameters six weeks post-treatment. CONCLUSIONS Infection with S. haematobium influenced the normal levels of certain blood constituents and treatment with praziquantel normalized the physiological conditions. According to WHO guidelines, regular mass treatment with praziquantel and health education are recommended as part of the primary health care programme in areas of hight prevalence and intensity of S. haematobium infection.